Method and apparatus for exploring subterranean strata



July 10, 1928.- 1,676,847

T. ZUSCHLAG METHOD AND APPARATUS FOR EXPLORING SUBTERRANEAN STRATA Fil ed .Jan. -1a, 1927 .1 9351" 1 1 111 11 E11 E11 ll EAST 111 11 111 111 VIII WEST 1 i fil & I H m N YY SOUTH IN VENTOR.

fieodar inc/Play 7 ATTORNEg Patented July 10, 1928.

'IPA'I-T'ENT; OFFICE.

rmzonon zuscnnse; or imw YoBK, N. Y., AssIeNon To run 'rlunuc Centenarian, ommw YOBK,-N. 1,1 conromnonor maw Yonx.

uarnon m APPARATUS roa' axr oRme-sunrnamnm arms-4..

7' Application-flea January is; 1927. Serial No. 1ai',sao.

My invention relates to the art of detecting and locating subterranean ore-bodies, salt domes, faults,'ete., by generating subtcrraneanelectrie currents'and observing the characteristics of their resulting..-..magnetic fields at different points. 7

It 'is known that the characteristics of such tields will he modified by variations in conductivity and permeability of the various ores, rocks. etc. of the earths crust, and that these modifications may be detected by suitable instruments under, at or below the surface of the earth and may be interpreted With respect to their geological.meaning.'

Several methods and apparatus have been proposedto take advantage of ,these facts in investigating subterranean strata. ln nearlyall of he known methods the use of alternating current hasheen proposed but in every instance the determination of the. magnetic field characteristic is limited to the factors of field direction and field intensity. Noattempt is made to determine the phase angle Variations in such fields. Taking into consideration the fact that all alternating current computations depend upon direct or indirect knowledge'of phase angles. it may be safely asserted that the attempt to interpret modifications in the characteristics of an alternating magnetic earth field, without taking into consideration the question of phase angles. must resultin incomplete and even incorreetdata.

The object of my invention is to provide a novel method and apparatus whereby-the complete characteristic of an alternating magnetic earth field may be determined with reference to direction. intensity and phase angle at any point within the field.

In the accompanying drawings I have illustrated diagrammatically how' the invention may be carried out, but it will be apparent to anyoneskilled in the art that the underlying principle may be embodied in 5 other forms and that variations in detail of the system illustrated may be made without departing from the spirit of the invention.

Fig. 1 illustrates diagrammatically a suitable apparatus for generating subterrenean alternating earth currents; Figs. 2 and 3 illustrate, respectively, two .stages of the method and apparatus employed in observing the characteristic of the magnetic field; and Fig. 4 is a chart showing one system of choosing the points of observation.

and. because the The alternatin magnetic field may be produced in any hn'own or suitable manner,

,for example, as indicated in'Fig. 1, but this does not constitute a part of the present invention, except I in combination with the method of observing the magnetic field. .As

shown in Fig. 1, (and more fully described and claimed in my co-pending application filed May 15, 1925, Serial No. 30,478), a generator 1 is in circuit with primary coil. 2 of a transformer having two secondary coils 1-). 4. Thecoils 3, 4 are oppositely wound and connected by wires 5, 6 and 7, 8 with grounded electrodes 9, 10 and 11, 12.. By' means of variable resistances 13, 14the'currents in the two circuits may be made equal ortions' of the circuits above grounder-e or most of their lengths parallel and close together, and the currents are; equal and opposite, the disturbing'influeuee of the local ficldmay be reduced to a negligibleminimum. Owing, however, to the positions of the grounded electrodes, the subterrenean paths will be of unequal length and. hence, of unequal resistance, and the ground currents will not be equal. A differential current will result on the basis of which the observations hereafter described, may be made.

Referring now to Fig. 2, 21 and 22 represent coils of large diameter having several hundred turns and being separated by a distance-from 25 to 500 feet. The coils may be turned in any direction and may be successively connected to an amplifying device 23 and a telephone 24. When the coilsare located in an alternating magnetic field of audio-frequency, alternating currents of the same frequency will be induced inthe-coils, amplified in the amplifier 23 and made audible in the telephone 24.

- The E. M. F.s induced in the coils depend upon the number of turns and the geometrical dimensions of the coils as 'well as the frequency and intensity of the magnetic field mo in the directions of the axes of the coils. .A turning of, the COIlS causes an mcrease orrdccrease of the induced E. M. F. yet normally it is notpossible to determine by such opera.-

tions the exact'directions of the maximum maximum and minimum field directions, in- 1151- coil 22, reference is'made to tensities and phase angles at *location IIof Fig? 3. The coils 22 and 21, the latter at location I,-are

-disposed ,with their axeslparallel to -any chosen direction and are connected by: double wires 25 and26 to the amplifying device 23, telephone-24-' and -.a measuringiarrange ment consisting of variable condensers 27' and 30 and variableresistances-'28 and' 29; The free ends of wires 25 and 26 are connccted to the inner endsof variable resistances 28 and 29. Ordinarily a sound will be heard in the telephone, but by varying the condensers 27 and 30 and the resistances 28 and 29, 'it is possible, to balance this bridge arrangement so that the sound will completely disappear;

Provided the coils 21 and 22 have the same.

number of turns and the same dimensions, the following proportion is valid:

Eu n l where E and E represent the E. M. F.s induced "in the coils bythe field strengths H, and H If R and index reprecuts t-hccomplexalterna-ting current resistance of the instrument corresponding to the index numher, the followingequat on isvalid for the balancing point:

i( 27+ 2a) A 30) -R21+R25+R21+R .R22+R26+Ra+Ba or, using the first proportion,

This equation may be transformed into the expression where A 11 is the quotient of field intensity and a n the phase angle difference for locations I and II, while the letters e and j represent enerally known .mathematieal symbols; he values of A 11 andam can be obtained by well-known methods of computation.

These calculations'require the knowledge of the frequency of the inducedcurrents.

' Because the frequency can be determined by i fter having thus determined the intensity-quotient and phase angle difference for certain coil positions at locations I and II, the same operation can be repeated for any 0f cdil 21-ii1tlocation Isle:thins-unchanged.

Provided the intensity quotients and phase angle differences are. determined for -two directions perpendicular .,to each other at location II, the exactmax'ii'num and minimum fiGld'dlL'eQtlOIl for the plane of the two measured directions as well as the ,maxuuum and minimum intensity quotients and, phase angle difl'erences for this plane with refer-r ence tothe field strengthat location I can be derived fromthese values by well known computationmethods If only the knowledge of the exact maximum-and minimum field direction isrcquired: coil 21 may be moved to locationlI, placed" in a perpendicular position to coil 22 and the same operation and calculation be carried out. I i In order to compute the absolute'maxi- .mum andminimiim'directions for location II and the absolutemaximum and minimum field intensity quotients and phase angle differences in-- reference to a certain field strength at location I or II, it is necessary to determine the intensity quotient and phase angle difference for a third po ition of coil 22v verticalto the'two other positions.

The same operations and'computations can further be repeated for location II- andanyother location III, coil 22 at location II now "beingplaced in one of the previously investigate positions and coil 21, now at location III,,placed successively into two or three positions perpendicular to each other.

The balance equation can then be expressed as: I

HIII n n1 In where H represents the field strength at location III and'A i and an 'the intensity quotient and phase angle difference with reference to locationII and III.

computed valuesit is necessary to refer all results to a standard field strength and preferablyto use the same as the u-nitfor all comparisons. For example choosing the field strength H as standard and unit, I can transform;

I HI

where Inf rrn in and at In a! u an m 9 It is easily comprehensible that by using the. measuring arrangement and by repeati'ng this method the complete characteristic of an alternating magnetic field maybe de- -In order to compare the measured and v a H m I: m Into :"=A In m 11! termined at any point within the field. For example, 1n order to investigate/a certain} are measuredas shown in Fig. 4. If the pro- -area a number of profiles or cross sections to each other at one or two calculating the intensity .files are measured from east to west it is necessary to run one profile from north to south in order to connect the difierent east and west profiles with each other; and vice versa.

After havin thus determined the field characteristic or different points, lines of equal maximum and minimum magnetic field direction, intensity and phase angle, the lattertwo-with reference to the chosen standard intensity and phase angle, may be interpolated, plotted and their irregularities interpreted with reference to their probable geological meaning.

I claim:

-1. The method of ascertaining the characteristics of an alternating magnetic field, which consists in receiving induced currents in-two or three directions perpendicular to each other at one or two locations in said field, measuring the frequency of said currents, balancing said currents by means of variable impedance devices of known values, uotients and phase angle differences for said irections means of the data thus obtained and computing from these values the maximum and minimum field directions and maximum and minimum field intensity quotients and phase angle difierences for the points of observation.

2. The method of investigating subterranean strata which consists in generating subterranean alternating currents, receivinginduced currents from the resulting magnetic field in two or three directions perpendicular locations in said .field,

servation,

measuring the frequency of said currents, balancing said induced currents by means of variable impedances of known value, calculating intensity quotients and phase angle differences for said directions by means, of the data thus obtained, computing from these values the maximum and minimum field direction and the maximum and minimum field intensity quotients and phase angle difierences for the points of oband repeating the foregoing steps with reference to a ties, thus accumulatmg data from which the magnetic field as a whole may be charted.

3. Apparatus for exploring an alternating magnetic field comprising two exploring antennae, a common indicator of alternating current in circuit with said. antenna, a bridging device shunted around said indicator and having its bridging elements connected to .the main circuit between said attennac, said bridgingidevice havin variable impedance elements of known va ue in its branch "arms, whereby one or both of the antennae circuits may be tuned to the frequency of the induced currents for the highest possible degree of sensitiveness and whereby the effects of the said currents may be balanced in said indicator and the intensity quotient and phase angle difference of the magnetic fields inducing said currents may be calculated from. the known constants of the circuits.

The forg going specification signed at New York, N. this 11 day of January, 1927.

THEODOR ZUSCHLAG;

plurality of other locali- 

